One known type of static read/write memory cell is a high-density static random access memory (SRAM). A static memory cell is characterized by operation in one of two mutually-exclusive and self-maintaining operating states. Each operating state defines one of the two possible binary bit values, zero or one. A static memory cell typically has an output which reflects the operating state of the memory cell. Such an output produces a "high" voltage to indicate a "set" operating state. The memory cell output produces a "low" voltage to indicate a "reset" operating state. A low or reset output voltage usually represents a binary value of zero, while a high or set output voltage represents a binary value of one.
A static memory cell is said to be bistable because it has two stable or self-maintaining operating states, corresponding to two different output voltages. Without external stimuli, a static memory cell will operate continuously in a single one of its two operating states. It has internal feedback to maintain a stable output voltage, corresponding to the operating state of the memory cell, as long as the memory cell receives power.
The two possible output voltages produced by a static memory cell correspond generally to upper (V.sub.cc internal-V.sub.T) and lower (V.sub.ss) circuit supply voltages. Intermediate output voltages, between the upper (V.sub.cc -V.sub.T) and lower (V.sub.SS) circuit supply voltages, generally do not occur except for during brief periods of memory cell power-up and during transitions from one operating state to the other operating state.
The operation of a static memory cell is in contrast to other types of memory cells such as dynamic cells which do not have stable operating states. A dynamic memory cell can be programmed to store a voltage which represents one of two binary values, but requires periodic reprogramming or "refreshing" to maintain this voltage for more than very short time periods.
A dynamic memory cell has no internal feedback to maintain a stable output voltage. Without refreshing, the output of a dynamic memory cell will drift toward intermediate or indeterminate voltages, resulting in loss of data. Dynamic memory cells are used in spite of this limitation because of the significantly greater packaging densities which can be attained. For instance, a dynamic memory cell can be fabricated with a single MOSFET transistor, rather than the six transistors typically required in a static memory cell. Because of the significantly different architectural arrangements and functional requirements of static and dynamic memory cells and circuits, static memory design has developed along generally different paths than has the design of dynamic memories.
A static memory cell 10 is illustrated in FIG. 1. Static memory cell 10 generally comprises first and second inverters 12 and 14 which are cross-coupled to form a bistable flip-flop. Inverters 12 and 14 are formed by first and second n-channel pulldown (driver) transistors N1 and N2, and first and second p-channel load (pullup) transistors P1 and P2. Transistors N1 and N2 are typically metal oxide silicon field effect transistors (MOSFETs) formed in an underlying silicon semiconductor substrate. P-channel transistors P1 and P2 can be thin film transistors formed above the driver transistors or bulk devices.
Driver transistors N1 and N2 have respective source regions 66 and 68 tied to a low reference or circuit supply voltage, labelled V.sub.ss, and typically referred to as "ground." Driver transistors N1 and N2 have respective drain regions 64 and 62, and respective gates. Load transistors P1 and P2 have respective source regions 78 and 80 tied to a high reference or circuit supply voltage, labelled V.sub.cc, and have respective drain regions 70 and 72 tied to the drains 64 and 62, respectively, of the corresponding driver transistors N1 and N2. The gate of load transistor P1 is connected to the gate of driver transistor N1. The gate to load transistor P2 is connected to the gate of the driver transistor N2.
Inverter 12 has an inverter output 20 formed by the drain of driver transistor N1. Similarly, inverter 14 has an inverter output 22 formed by the drain of driver transistor N2. Inverter 12 has an inverter input 76 formed by the gate of driver transistor N1. Inverter 14 has an inverter input 74 formed by the gate of driver transistor N2.
The inputs and outputs of inverters 12 and 14 are cross-coupled to form a flip-flop having a pair of complementary two-state outputs. Specifically, inverter output 20 is coupled to inverter input 74 via line 26, and inverter output 22 is coupled to inverter input 76 via line 24. In this configuration, inverter outputs 20 and 22 form the complementary two-state outputs of the flip-flop.
A memory flip-flop such as that described typically forms one memory element of an integrated array of static memory elements. A plurality of access transistors, such as access transistors 30 and 32, are used to selectively address and access individual memory elements within the array. Access transistor 30 has one active terminal 58 connected to cross-coupled inverter output 20. Access transistor 32 has one active is terminal 60 connected to cross-coupled inverter output 22. A pair of complementary column or bit lines 34 and 36 are connected to the remaining active terminals 56 and 54 of access transistors 30 and 32, respectively. A row or word line 38 is connected to the gates of access transistors 30 and 32. In the illustrated embodiment, access transistors 30 and 32 are n-channel transistors.
Reading static memory cell 10 requires activating row line 38 to connect inverter outputs 20 and 22 to column lines 34 and 36. Writing to static memory cell 10 requires complementary logic voltage on column lines 34 and 36 with row line 38 activated. This forces the outputs to the selected logic voltages, which will be maintained as long as power is supplied to the memory cell, or until the memory cell is reprogrammed.
In semiconductor processing, there is a continuing desire to make circuits denser, and to place components closer and closer together to reduce the size of circuits. However, certain processing steps employed in manufacturing static memory cells such as the static memory cell shown in FIG. 1 result in some undesirable variations between desired results and actual results in the manufacturing process. For example, there are precision limits inherent in photolithography. Another process that results in some undesirable variations between desired results and actual results is called LOCOS isolation (for LOCal Oxidation of Silicon). LOCOS isolation is a common technique for isolating devices.
Implementing a static memory cell on an integrated circuit involves connecting isolated circuit components or devices, such as inverters and access transistors, through specific electrical paths. When fabricating integrated circuits into a semiconductor substrate, devices within the substrate must be electrically isolated from other devices within the substrate. The devices are subsequently interconnected to create specific desired circuit configurations.
LOCOS isolation involves the formation of a semi-recessed oxide in the non-active (or field) areas of the bulk substrate. Such oxide is typically thermally grown by means of wet oxidation of the bulk silicon substrate at temperatures of around 1000.degree. C. for two to six hours. The oxide grows where there is no masking material over other silicon areas on the substrate. A typical masking material used to cover areas where field oxide is not desired is nitride, such as Si.sub.3 N.sub.4.
However, at the edges of a nitride mask, some of the oxidant also diffuses laterally immediately therebeneath. This causes oxide to grow under and lift the nitride edges. The shape of the oxide at the nitride edges is that of a slowly tapering oxide wedge that merges into a previously formed thin layer of pad oxide, and has been termed as a "bird's beak". The bird's beak is generally a lateral extension of the field oxide into the active areas of devices.